At present, most high speed (1 Gbps or more) electrical communication over cables is done through differential cables. Differential communication is considered as the best way to achieve sufficient Electro Magnetic Compatibility (EMC), being low radiative emissions at the transmit side and good bulk current immunity at the receive side. Many examples of commercialized technologies exist, like, USB (Universal Serial Bus), HDMI (High Definition Multimedia Interface), DisplayPort, Serial ATA (Advanced Technology Attachment), etc. . . . However, differential connectors and cables are more expensive than single ended coaxial cables and connectors. Furthermore, at high bit rate, differential cables often generate intra-pair skew. This leads to wave coupling in the cable between differential and common modes, and to final destruction of the transmitted signals. Skew compensation circuits can only effectively compensate very limited skew at receiver's end due to the destructive effect of the coupling. A single ended cable like a coaxial cable cannot have any skew problems due to its single ended nature.
Another merit of single ended cabling systems is their relatively well-known characteristic impedance. E.g. one can easily find on the market an RG174 coaxial cable with a 50Ω impedance having a tolerance of +/−2Ω, whilst for a differential system, a typical specification is 100Ω impedance with a tolerance of +/−10Ω. The higher relative uncertainty on the characteristic impedance makes a differential cable a less attractive candidate for bidirectional communication.
Single ended transmitters typically comprise a single ended driver, with or without source termination. With source termination, there exist drivers having this termination on chip and others assume termination on the Printed Circuit Board (PCB). For digital non-return-to-zero (NRZ) communications, single ended drivers can be as simple as a single inverter or buffer. At high speed, this type of system is highly unbalanced, leading to unwanted electromagnetic radiation, even when the transmit system is in a shielded box.
Better quality are the differential transmitters, for example the video cable drivers for broadcasting equipment, whereby a balanced differential driver typically is connected with two single ended coax connectors at the edge of a PCB. When two cables are effectively connected to these coax connectors, there is a well balanced system that shows sufficient low radiation for this digital broadcasting application. These applications are further low demanding on EMC levels. For consumer and certainly for automotive applications the EMC specifications are more difficult to reach.
When the chip also serves as a receiver for high speed signals, it can further be the case that there is frequency dependent signal loss in the coaxial cable. This can be compensated for, e.g. by the use of a fixed or self adaptive equalizer. The difficulty here is that the received high frequency components may become very small with respect to the initial launched signal, and by being small it can easily become a victim of injected bulk current. The latter can be a result of electromagnetic interference applied close to the receive end of the coaxial cable.
U.S. Pat. No. 6,426,970B1 shows a bidirectional splitter for communication over a common coaxial cable and also many prior art circuits. Little or no attention is given to the design for good EMC of circuits and PCBs.